This invention relates to magnetic apparatus for the testing of metal bodies such as shafts, tubes, strips, etc., to detect and locate defects therein in general, and more particularly to an improved device of this nature which induces a traveling field such as a rotating field in the test piece.
Devices of the type which induce eddy currents in a metal test object, often in connection with a relative motion between the device for testing and the transducer of the device, are known. With a device of this nature, a fault in the test piece will result in an alteration of propagation of the eddy current. This fault can then be electrically detected to provide an appropriate output indicating the presence of a fault. Althrough these devices operate well under some circumstances, they suffer from certain disadvantages. Very often, the direction of relative movement between the device transducer and the test piece is in a direction parallel to the longitudinal axis of the test piece, particularly when this piece is a rod, drive shaft, or the like. As a result the sensitivity to measurement is not always as high as its desired. This is particularly true in the axial direction. However, objects of this nature very often have faults which have an axial extension. Thus, such faults only disturb the field a small amount and can pass by undetected.
In view of this, it is clear that there is a need for improved apparatus which is more sensitive to all types of faults and also which can provide additional information regarding the type and location of the fault.